Screen systems used in the coal industry and elsewhere to separate water from granular material being processed and to separate granular material according to size have one or more sieve decks, each of which comprises a plurality of removable and replacable modular screen sections supported on a base such as channel irons or other structural material. Modular screen sections of the prior art include a foraminated surface formed from plastic or stainless steel profile wires and surrounded by a frame formed from a durable plastic such as polyurethane reinforced with metal rods embedded in the plastic.
It is known in the prior art to make the modular screen sections of uniform cross-sectional dimensions and to releasably attach the abutting screen sections to the underlying base with locking pins. See, for example:
U.S. Pat. No. 3,980,555 issued Sept. 14, 1976 to Freissle for REPLACABLE SCREEN WITH FRAME; PA0 U.S. Pat. No. 4,141,821 issued Feb. 27, 1979 to Wolff for SCREENING DECK ASSEMBLY; and PA0 U.S. Pat. No. 4,409,099 issued Oct. 11, 1983 to Wolff for SIEVE DECK FOR SIFTING MACHINES.
The locking pins disclosed in these patents, and the locking pins in all prior art screen assemblies known to applicant, are intended to hold the abutting screen sections tightly against each other to prevent undesirable leakage of granular material between the abutting screen sections. Nevertheless, leakage does occur between the longitudinal ends of adjoining screen sections of the prior art when fine gauge material is being processed on screens formed with stainless steel profile wire as the sieve deck. It is known to put foam rubber over the juncture between the longitudinal ends of adjoining screen sections and anchor it in place with angle irons in an effort to control the leakage, but the prior art attempts to control the leakage have not been successful because the angle irons become loosened and the foam rubber deteriorates.
The locking pins in all prior art screen assemblies known to applicant do not extend above the surface of the screen. It is, in fact, accepted practice to specifically structure the prior art locking pins so they will not protrude above the surface of the screen. It has been thought that locking pins protruding above the surface of the screen will undesirably interfere with the flow of the granular material being processed.
It has been recognized in the prior art, however, that it is desirable to provide one or more dams extending across at least part of the path of the granular material being processed to control the flow of the granular material. Most prior art dams are formed integrally with the screen sections, but U.S. Pat. No. 4,141,821 to Wolff shows a movable dam with its own pins.